Devices or elements of bistable effect which when suitably incorporated into larger systems can function as data processors for executing operations or for storing information are already known, and are frequently used in constructing microprocessors for information technology or telecommunications use.
In constructing such devices, materials which demonstrate the bistable effect in certain of their characteristics are used. For example, a description of some of these materials is given in "Optical bistability: Controlling light with light", H. M. Gibbs, Academic Press 1985, page 120.
Research on materials of bistable effect have lead to the preparation of new materials and improvements in the characteristics of already known materials. The materials studied are not always "new". In this respect, they may have been known and been used for some time, but are still studied in order to discover new aspects of their physical behaviour which would allow the development of new devices offering interesting applicational developments, as will be apparent from the description of the device of the present invention.
It is opportune to briefly consider the materials which satisfy the aforesaid requirements and are currently proposed for known applications. These include inorganic semiconductors such as selenium oxychloride, which is able to maintain active ions such as neodymium (Nd.sup.+3) in solution and present non-linear optical effects. This type of material provides an optical action with low threshold values. This is due to the presence of a very narrow fluorescent emission line similar to that of YAG (yttrium-aluminium-garnet) crystals.
Liquids providing non-linear optical emission are also known. These liquids are dye solutions, such as obtained by dissolving substances such as carbocyanine tetrafluoride, acridine red etc. in ethyl alcohol.
Materials of crystalline structure and glass structure are also known, these materials having become accepted as basic components in microelectronic semiconductor devices for the manufacture of logic circuits, microprocessors, memories and other information technology equipment. Materials of crystalline structure include mainly silicon, germanium, ruby etc., and also compounds such as gallium arsenide (GaAs), cadmium tetrafluoride (CdTe) and others. These materials must be available in crystalline form with a high degree of purity to be subsequently subjected to controlled additions of impurities (doping) in order to form semiconductor materials with precise electrical characteristics.
This observation emphasizes the importance of the degree of purity and the homogeneousness of "doping" in the production of materials, these characteristics even today being obtained only by complex and costly technological processes, with sometimes poor reproducibility of results, particularly for compounds.
Certain types of glass used as amorphous semiconductors have recently gained particular importance. The glasses used are mixtures of several compounds, of which in the current state of the art chalcogenide glass has suitable characteristics for constructing monostable and bistable switches for use in electronic processors and in industrial process control.
These amorphous semiconductors are still not widely used because they require the development of new techniques which have to ensure optimization of their composition, their reproducibility and their stability with time or variations in environmental conditions (thermal cycles).
The mechanism used to activate the bistable effect in the aforesaid materials is a thermally induced variation (thermal fluctuation) in the real part or in the imaginary part of the refractive index. It is known that thermal fluctuations alter the material density and consequently its refractive index.
In the specific case of semiconductor materials the bistable effect is generated by the energy band filling effect and by the excitonic band selection effect.
The excitonic bands correspond to the presence of excitons in the material, i.e. the existence of excited states which can concern either the entire material or a defined region of it, and can also propagate through the molecular structure of the material to transport energy without transporting the electric charge.
A lattice structure of a binary compound such as gallium arsenide (GaAs) has reached a power level of 10 milliwatts (mw) with a switch-up time of one picosecond (1 ps) and a switch-down time of 40 nanoseconds (40 ns) as specified in "Optical bistability: Controlling light with light", H. M. Gibbs, Academic Press 1985, page 305.
However, these considered materials have an exciton-creating optical absorption band width which is too narrow and a manufacturing cost which is too high, making the creation firstly of a material and then of an electronic device improbable. In addition to this, their technological limitations are still very severe. In this respect many problems concerning the homogeneousness of "doping" and the formation of ohmic contacts still have to be solved. Many techniques have been suggested for this latter problem, but the reproducibility of the results is modest.
It must also be noted that for most electronic or microelectronic applications a high switching speed corresponding to the aforesaid values is not required. The industrial production of doped crystals is difficult compared with the preparation of the polymer material of the present invention. Said polymer material enables an optical device to be formed which can operate at lower power levels than the doped glass used as amorphous semiconductors.
Said polymer material can also be integrated with other identical polymer material whereas the cells of the aforesaid liquids cannot be mutually integrated.